Continuous electron injector for crossed-field switch tubes

ABSTRACT

Continuously energized field emission device injects electrons into the interelectrode space of a crossed-field switch tube to eliminate the dependence upon statistical electrons for initiation of a discharge.

United States Patent [:91

Lutz et al.

[ June 17, 1975 l CONTINUOUS ELECTRON INJECTOR FOR CROSSED-FIELD SWITCH TUBES [75] Inventors: Miehael A. Lutz, Malibu; Robert Holly, Los Angeles, both of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Sept. 23, 1974 {21] Appl. No.: 508,389

[52] U.S. Cl. 313/157; 313/161; 313/162;

313/193 [51] Int. Cl. H0li 23/10 [58] Field of Search 313/157, 161, I62, 198

[56] References Cited UNITED STATES PATENTS 3,714,510 1/1973 Hofmann .1 313/180 X Primary Examiner-R. V. Rolinec Assistant ExaminerDarwin R. Hostetter Attorney, Agent, or FirmAllen A. Dickc, Jr.; W. H. MacAllister [57] ABSTRACT Continuously energized field emission device injects electrons into the interelectrode space of a crossedfield switch tube to eliminate the dependence upon statistical electrons for initiation of a discharge.

3 Claims, 2 Drawing Figures CONTINUOUS ELECTRON INJECTOR FOR CROSSED-FIELD SWITCH TUBES BACKGROUND OF THE INVENTION This invention is directed to a continuous fieldemission electron source for directing electrons into the interelectrode space of a crossed-field switch device.

Crossed field switching devices have been known for many years as laboratory curiosities, as shown in Penning U.S. Pat. No. 2,182,736; and as simple low power devices, as shown in Boucher U.S. Pat. Nos. 3,215,893 and 3,215,939 and Wasa U.S. Pat. No. 3,405,300.

Only recently, however, has the utility of such a switching device in high voltage, high current DC applications been recognized, because careful design is necessary for employment of the crossed-field device concept in high current and high voltage situations. Examples of such use are found in Kenneth T. Lian U.S. Pat. No. 3,534,226 and Gunter A. G. Hofmann and Ronald C. Knechtli U.S. Pat. No. 3,538,960.

When onswitching a crossed-field switch device, both an electric field and a magnetic field are applied so that breakdown of the interelectrode gas can take place. This requires an initial electron to start the avalance. Such electrons are always present on a statistical basis due to random cosmic ray events. G. A. G. Hofmann U.S. Pat. No. 3,7 l4,510 also described the onswitching conditions and employs a low level keep alive plasma to eliminate the wait for the initial electron. Another approach is to use a radioactive source. The structure of this invention provides field-emitted electrons injected into the interelectrode space. The patents to which reference is made in this specification are incorporated herein in their entirety by this reference.

SUMMARY OF THE INVENTION In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a continuous electron source for crossedfield switch tubes, the source being a field-emitting device secured to inject electrons into the interelectrode space of the crossed-field switch tube.

It is thus an object of this invention to provide an eco nomic, reliable and safe device of long life for injecting electrons into the interelectrode space to thus reliably permit the ignition of crossed-field switch tubes. It is another object to reduce the statistical time lag to less than one microsecond so that precise ignition with low (subrnicrosecond) shot to shot variation in firing is possible.

Other objects and advantages of this invention will become apparent from a study of the following portion of the specification, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side-elevational view of a crossed-field switch tube, with parts broken away and parts taken in section, showing the interelectrode space and showing the electron device secured thereto.

FIG. 2 is an enlarged section through the electron injector device.

DESCRIPTION In order to fully appreciate the electron injector device, some understanding of the crossed-field switch device 10 is required. It comprises housing 12 which is carried upon bottom flange 14. Bottom flange 14 is, in turn, mounted upon base flange l6 and they are secured together to provide a tight seal. Base flange l6 stands upon foot 18 for supporting the switch device structure. Furthermore, foot 18 can act as a vacuum connection for drawing a suitable vacuum on the interior of the housing 12 and then letting into the housing the desired gas (e.g., helium or hydrogen, including its isotope deuterium) at the required pressure. Housing 12, together with bottom flange 14, serves as a suitable vacuum tight envelope. Cathode 12 is metallic and can be made of stainless steel. The cathode is connected to the foot 18, such as by metallic continuity. Thus, foot 18 provides one of the electrical connections to the switching device 10. Cathode 12 may have an axial slot to prevent the circumferential circulation of current during switching transients when the axial magnetic field changes with time.

Anode 26 is of cylindrical tubular construction and is positioned concentrically with cathode 12 to provide a radial space therebetween having the dimension d. The radial space d is substantially equal at all facing positions of the anode and cathode. Housing 12 has a top cap 28 upon which anode 26 is positioned. The anode is maintained in position by employing anode cap 30 which is secured to the cylindrical anode 26 and, in turn, carries mounted stud 32. Mounting stud 32 provides both mechanical support by being secured to housing cap 28 and provides electrical continuit through the cap by electrical connector 34. Preferably, anode cap 30 is spaced below top cap 28 and connector 34 passes through insulative mounting stud 32 so that connector 34 and the entire anode are electrically separated from the housing. Alternatively, top cap 28 can be of insulative material.

Anode 26 may be perforated so that the interior space thereof serves as a gas volume to supply gas to the interelectrode space. Furthermore, gas supply means can be provided interiorly of the anode to supply gas as it is consumed by glow discharge in the interelectrode space. Both of these concepts are taught in Hofmann and Knechtli U.S. Pat. No. 3,558,960. The maintenance of interelectrode space gas pressure is disclosed in more detail in that patent.

Magnet 36 is positioned on the exterior of housing 12 and in such a manner as to provide magnetic lines of force in the interelectrode space which are substantially parallel to the axis of the electrodes of switching devide 10 over at least part of the electrode length. Magnet 36 illustrated as being an electromagnet and such is prered so that the magnetic field can readily be switche on and off. The power supply to magnet 36 is preferably of such nature as to provide for rapid turn on and off of the field. Its strength is such as to provide a field of ab'out gauss for off-switching alone and about 1 kilo gauss for on-switching against high voltages (up to 100 kv).

Once a glow discharge is established and current is flowing, off-switching is accomplished by reducing the magnetic field strength to a point where cascading ionization cannot continue (typically to less than 50 gauss). Thus, condition ceases. This is explained in considerably more detail in the Hofmann and Knechtli U.S. Pat. No. 3,558,960 mentioned above.

In the absence of an applied magnetic field, the electrical breakdown of a gas at a pressure p, contained between two electrodes of spacing d, is governed by the Paschen curve for the specific gas in use. To remain in the nonconducting state under a given applied voltage, it is necessary to adjust the pd product of a crossed-field switch to fall to the left of the Paschen curve for that specific device. It is difficult to initiate a crossed-field discharge if the pressure is much below 50 millitorr, so this sets an upper limit on the interelectrode spacing of about 4 centimeters for helium. The minimum electrode spacing is fixed by the requirement that the field emission level will not be high enough to initiate a discharge at the maximum voltage level. Several experiments indicate that a spacing of at least 1.5 centimeters is necessary to hold off l kilovolts. Thus, for a tube to remain in the nonconducing state under the influence of an applied high voltage, the interelectrode space must be kept between well-defined limits. The upper limit is determined by Paschen breakdown; and the lower limit, by field-emission-initiated breakdown.

When the device is in the nonconducting state, the electron mean-free path for an ionizing collision is much greater than d. Consequently, electrons which appear in the interelectrode space are lost at a rate that is too high for a gas discharge to be formed and sustained. Applying a sufficiently large magnetic force with one component perpendicular to the electric field, however, causes these electrons to remain in the interelectrode space for a considerably longer time. As a result, they can then make a sufficient number of ionizing collisions in spite of the low gas pressure, and a high density gas discharge can be formed (e.g., application of the magnetic field is, in effect, equivalent to increasing the pressure). For ignition to occur at an acceptably low value of magnetic field, it is necessary to employ a coaxial, cylindrical electrode arrangement because coaxial, cylindrical electrodes serve as the ideal trapping means.

The number of ampere turns necessary to trigger a crossed-field switch depends on the applied voltage and the field coil/electrode geometry. For a typical tube holding off 100 kilovolts, this is not excessive; it is only slightly greater than 2,000. The energy stored in the magnetic field at this level is approximately 1 to l0 joules.

lgnition jitter is defined to be the shot-to-shot variation in the time when current initiation occurs. This jitter must be within certain allowable limits which are determined by circuit and system requirements. In an idealized system, current initiation will occur at the time when the electric and magnetic fields reach the required values for ignition (e.g., there is electron trapping). in a real system, ignition can be delayed beyond this time, if there is a lack of initiatory electrons.

Electron emitter 50 is mounted on boss 52 secured on the exterior of cathode 12. Opening 54 through boss 52 permits access from the emitter 50 into the interelectrode space d. Emitter 50 comprises a flange 56 which is bolted down to boss 52 by conventional capscrews. Insulator 58 forms a collar around electrode 60. Electrode 60 has a sharp point 62 in chamber 62 recessed out of the interelectrode space a and directed into opening 54. Aperture disk 61 is positioned at the end of electrode 60. Electrode 60 is connected on its outer end through current-limiting resistor 64 to voltage source 66 which supplies an electric field between the sharp point 62 and anode 61. The sharp point is negative with respect to the anode. The sharp-pointed electrode emits electrons by field emission into the interelectrode space. The voltage is high enough for continuous field emission but not high enough to cause arcing breakdown.

lnitiatory electrons are provided by the field-emitting device (ionizer), positioned at the side or base of the crossed-field switch. This electrode carries a current in the range of l to 10 microamperes. Of this current, a small amount I is actually present in the active electrode area of the tube due to diffusion. Upon triggering, the tube current I is found to rise exponentially from the preionization level with a growth rate F of 1.5 X l0 /second according to the rule I l lt where:

I tube current 0 preionization current level P 1.5 X 10 /second growth rate r elapsed time from when conditions for ignition are correct i.e. voltage and B field are sufficient.

lf 1 is the current at the time that the anode voltage has collapsed, the time required for this to happen is the formative time I, and is given by:

1,: HT Fn 1/1,

Fluctuations in the preionization level (i.e., in 1,) lead to variations At, in the formative time, as follows:

where I, and 1,, are two different preionization levels. Such variations will show up as jitter in the anode fall time. For example, a fluctuation of will lead to a jitter time of l microsecond. A properly designed ionizer should not result in an I,,'/I greater than 10 over its lifetime and should therefore, contribute essentially nothing to the jitter of a crossed-field switch.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims:

What is claimed is:

1. A crossed-field switch tube having an electron emitter, said crossed-field switch tube comprising:

a substantially cylindrical tubular cathode, a substantially cylindrical anode positioned within said cathode and defining an annular interelectrode space therebetween, said anode and said cathode being electrically insulated from each other and being connectable to a source of electrode potential, a gas at subatmospheric pressure within said interelectrode space, the gas pressure being such, in relation with the interelectrode space dimension, that the conditions are outside the conductive area of the Paschen curve, a magnet positioned to provide a magnetic field in the interelectrode space so that an electron in the interelectrode space is caused to annularly spiral in the annular interelectrode space to provide cascading ionization and conduction between said anode and said cathode, said electron emitter comprising:

a field emitter in communication with said interelectrode space and insulated from one of said elec- 6 opening.

3. The cross-field switch tube of claim 2 wherein said opening is in said cathode and an insulated collar is secured to said cathode, said field emitter having a body positioned within said insulated collar and having a sharp edge extending beyond said collar for directing electrons toward said opening.

* t i it l 

1. A crossed-field switch tube having an electron emitter, said crossed-field switch tube comprising: a substantially cylindrical tubular cathode, a substantially cylindrical anode positioned within said cathode and defining an annular interelectrode space therebetween, said anode and said cathode being connectable to a source of electrode potential, a gas at subatmospheric pressure within said interelectrode space, the gas pressure being such, in relation with the interelectrode space dimension, that the conditions are outside the conductive area of the Paschen curve, a magnet positioned to provide a magnetic field in the interelectrode space so that an electron in the interelectrode space is caused to annularly spiral in the annular interelectrode space to provide cascading ionization and conduction between said anode and said cathode, said electron emitter comprising: a field emitter in communication with said interelectrode space and insulated from one of said electrodes so that upon application of an electrical potential to said emitter, electrons are emitted into said interelectrode space to reduce ignition time delay and jitter of said crossed-field switch tube.
 2. The crossed-field switch tube of claim 1 wherein a recessed opening is provided in the structure adjacent said interelectrode space and said field emission electrode is behind said opening and is directed at said opening.
 3. The cross-field switch tube of claim 2 wherein said opening is in said cathode and an insulated collar is secured to said cathode, said field emitter having a body positioned within said insulated collar and having a sharp edge extending beyond said collar for directing electrons toward said opening. 